In digital communication systems, timing synchronization is necessary for proper reception and decoding of information. Typically, a predetermined waveform, called a synchronization (sync) pattern, is combined with the information-bearing signal to be transmitted to enable the receiver to synchronize to the incoming signal. This combining usually takes the form of time interleaving or multiplexing. The sync pattern is usually designed to lie within the same frequency band as the data portion of the signal. The receiver uses a matched filter (whose impulse response is matched to that of the sync pattern), in combination with an energy or power detector and a peak detector, in order to derive timing information for proper reception. This process is illustrated in FIG. 1A. A receiver 101 receives the incoming signal from the channel. The instantaneous energy or power of a sync matched filter 103 output is then found by an energy/power detector 105. When the sync waveform arrives at the receiver 101, a peak occurs as is illustrated by signal 1A1. The presence and time of occurrence of the peak is determined by the peak detector 107, which then provides this timing information, signal 1A2, to the data detector 109 for proper decoding of data.
The duration of the energy peak of the sync matched filter output is on the order of the reciprocal of the bandwidth of the sync pattern. This is usually a desirable situation, since a shorter peak provides more precise timing information, and more precise timing is usually necessary for wider bandwidth data signals. In an implementation characterized by analog components, the continuous version of the waveform 1A1 is available for determination of the timing peak. In receivers implemented using digital signal processors (DSPs), only a sampled version is available. The sampling rate must be high enough for the peak detector to be able to recognize the presence and location of the peak. This may require a large amount of DSP processing power if the bandwidth of the sync signal is large, such as in the case of a wideband signal, which is wider in bandwidth than a voice signal. For instance, if the bandwidth of the sync signal is on the order of 1 MHz, the sync matched filter output may be on the order of 1 microsecond long. If four samples across that peak are necessary for proper operation of the peak detector, the sampling rate of the process must be on the order of 4 MHz. Such a high processing rate may require an enormous amount of DSP processing power and consequently a relatively enormous amount of electrical power. This amount of power would quickly drain a battery on a portable radio, which already has a relatively short battery life.
Such high processing power is unavoidable if DSP implementation is desired and if very accurate timing accuracy is needed. If the timing accuracy requirement can be relaxed, as is possible, for example, with the use of multi-channel modulation for the information portion of the signal, then the bandwidth of the sync waveform can perhaps be reduced to less than that of the data signal, thereby reducing the sampling rate and processing power required. This reduced bandwidth idea is illustrated in FIG. 1B. Unfortunately, this approach does not operate satisfactorily in environments (e.g., mobile radio channels) where the channel's frequency response is not flat across the band of interest (i.e., frequency selective channels). Generally the wider the signal bandwidth (the higher the data rate), the more likely it is that the channel will appear to be frequency selective. In such cases, a narrowband sync waveform may be so grossly attenuated that it can provide no useful timing information, even though other portions of the data signal may still be recoverable.
Accordingly, a method for synchronizing wideband signals characterized by reduced processing requirements, as well as robust performance in frequency selective channels, is required.